A great way to improve your pull-up strength is by doing
multiple sets of low reps. When most people do pull-ups
they will grab the bar and grind out as many reps as they
possibly can (usually not that many for most folks).
But rather then training to failure on a few sets, strive
to do 10 sets of 2 reps. The way it will go is you'll do
your first set of 2 reps. Rest 1 minute. Then do your
second set of 2 reps. Rest 1 minute. etc... until you
have completed 10 sets of 2 reps (i.e. 20 total pull-ups).
Even if you can't get all 10 sets of 2 reps at first,
just do as many sets of 2 reps as you can. Then for each
back workout push yourself to improve until you are able
to complete all 10 sets of 2 reps.
Once you can knock off 10x2 then add 1 rep to each set.
So you will work towards completing 10 sets of 3 reps.
Then 10 sets of 4 reps, etc. When you get to doing
10 sets of 5 reps (i.e. 50 total reps) keep the volume
close to the same and work on performing the reps with
fewer sets. (i.e. 9 sets of 6 reps, 8 sets of 7 reps,
7 sets of 8 reps, etc...)